1. Field of the Invention
The invention relates to an electrochemical secondary cell having a negative electrode containing lithium as active material, having a positive electrode containing a lithium manganese oxide spinel as active material and having a nonaqueous electrolyte.
The field of application of the invention therefore encompasses all secondary cells whose negative electrodes are composed of lithium metal, a lithium alloy or a carbon material capable of Li intercalation.
2. Description of the Related Art
The importance of lithium manganese oxide spinels for the development of novel rechargeable battery systems already emerges from a series of patent publications, for example U.S. Pat. No. 4,507,371, U.S. Pat No. 4,828,834, U.S. Pat. No. 4,904,552, U.S. Pat. No. 5,240,794. Representatives of this group of compounds which are particularly suitable as active electrode material crystallize in a spinel lattice having a cubic close-packed arrangement of the oxygen atoms, for example LiMn.sub.2 O.sub.4 and Li.sub.4 Mn.sub.5 O.sub.12. Extraction of the Li from LiMn.sub.2 O.sub.4 with a dilute mineral acid (H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, HNO.sub.3, HClO.sub.4 or HCl) or even by electrochemical means results in the formation of a .lambda.-MnO.sub.2. The latter has a defective spinel structure with a reduced lattice constant.
Other compounds such as LiMnO.sub.2 have an octahedrally distorted common-salt lattice. In Li.sub.2 MnO.sub.3, the oxygen atoms form a cubic close packing in which Li.sup.+ and Mn.sup.4+ ions distribute themselves over the octahedral gaps in an alternating layer sequence.
The compounds LiMn.sub.2 O.sub.4 and Li.sub.4 Mn.sub.5 O.sub.12 mentioned are stoichiometric spinels (German Offenlegungsschrift 4119944 ). LiMn.sub.2 O.sub.4, which is already frequently used as cathode in rechargeable cells and batteries, is produced by causing a lithium salt or lithium hydroxide to react with a manganese oxide at temperatures above 750.degree. C.
In the cubic oxygen lattice of the LiMn.sub.2 O.sub.4 spinel, the Li.sup.+ ions are situated in tetrahedral gaps and the Mn.sup.3+ /Mn.sup.4+ ions in octahedral gaps. The lattice constant is 8.25 .ANG..
The more Li-rich spinel Li.sub.4 Mn.sub.5 O.sub.12 can also be described by the formula Li[Li.sub.1/3 Mn.sub.5/3 ]O.sub.4 both stoichiometrically and in regard to the atomic distribution. Like .lambda.-MnO.sub.2, it contains only 4-valent manganese. The crystal symmetry is also cubic, but, because of the substitution of the larger Mn.sup.3+ ions (r=0.645 .ANG.), which form half of the manganese in LiMn.sub.2 O.sub.4, by the smaller Mn.sup.4+ ions (r=0.530 .ANG.) the lattice constant is substantially reduced to 8.17 .ANG..
Although the substitution of Mn.sup.3+ by Li.sup.+ in accordance with 3 Mn.sup.3+.fwdarw.Li.sup.+ +2 Mn.sup.4+ results in a higher Li content in the Li.sub.4 Mn.sub.5 O.sub.12 in total, when compared with that of the LiMn.sub.2 O.sub.4, the concentration of the electrochemically active Li is not increased because only the lithium situated in the tetrahedral gaps is accessible under normal potential conditions to an electrochemical intercalation and deintercalation. This is because the additionally intercalated lithium shares the octahedral gaps with the manganese and is immobilized at those points.
It is also known that lithium batteries can be operated at 4V and at 3V with lithium manganese oxide compounds as cathode material. If the spinel LiMn.sub.2 O.sub.4 is the discharge product, it can be recharged by Li-deintercalation, as a result of which the average oxidation level of the Mn increases from 3.5 to 4 (in the .lambda.-MnO.sub.2). Such a cathode material, corresponding to the general composition Li.sub.1-x Mn.sub.2 O.sub.4 where O&lt;x&lt;1, can be used for a 4V system. The spinel structure is retained over the entire composition range and the Mn/O ratio is constant.
In the case of a 3V system, the spinel LiMn.sub.2 O.sub.4 is the cathode material of the general composition Li.sub.1+x Mn.sub.2 O.sub.4, where 0&lt;x&lt;1, in the charged state. Its discharge (intercalation of Li) ultimately results in Li.sub.2 Mn.sub.2 O.sub.4 with tetragonal crystal symmetry. The electrode material consequently becomes a 2-phase material, with the disadvantageous consequences that the reversibility of the electrode reaction decreases (reduced resistance to cycling) and that the Mn ions present in the Li.sub.2 Mn.sub.2 O.sub.4 phase exclusively in the 3-valent state tend to disproportionate in accordance with 2 Mn.sup.3+.fwdarw.Mn.sup.4+ +Mn.sup.2+, in which process they dissolve in the electrolyte as Mn.sup.2+.
In order to stop, at least partially, the tetragonal distortion of the spinel structure, German Offenlegungsschrift 4328755 has already proposed a stoichiometry Li.sub.1+x Mn.sub.2-x O.sub.4+.delta., where 0.ltoreq.x&lt;0.33 and 0.ltoreq..delta.&lt;0.5, for the cathode material, in which connection the variable parameters x and .delta. should be chosen so that, in the discharged state in which it is used, the material should still just have a cubic symmetry (spinel lattice) and the mean degree of oxidation of the Mn is not below 3.5.
In contrast to the abovementioned "oxygen-rich" spinels is an oxygen-deficient spinel Li(Li.sub.1/3 Mn.sub.5/3)O.sub.4-.delta., which was characterized by M. N. Richard et al. (Solid State Ionics 73 (1994), 81-91). Here an increasing .delta. (oxygen deficiency) is responsible for an average degree of oxidation of the Mn which decreases from 4 toward 3.5. As a result, Li can be deintercalated, with simultaneous oxidation of Mn.sup.3+, with the result that such a spinel can also be used as a reversibly functioning cathode material.
Common to all the known Li-rich spinels of the type Li.sub.1+x Mn.sub.2-x O.sub.4 and all oxygen-rich spinels of the type Li.sub.1-x Mn.sub.2-x O.sub.4+.delta. is a relatively low true density and apparent density with large globulitic crystallites and relatively high BET surface (&gt;3 m.sup.2 /g) . The capacities achievable with the cathode materials vary between 120 mAh/g and 10 mAh/g, depending on the value of x.
Compounds of the type Li.sub.1+x Mn.sub.2 O.sub.4 have, as the stoichiometric Li.sub.2 Mn.sub.2 O.sub.4 is approached, its tetragonal crystal structure (two-phase region) and are therefore no longer in the region of existence of cubic spinels from a crystal-chemistry point of view. In addition, these Li-rich 3V materials, in particular Li.sub.2 Mn.sub.2 O.sub.4, are already unstable in air and are moisture-sensitive.
In most cases, an improved high temperature storage capability and a satisfactory cycling stability are acquired only at the expense of low capacities.
The object of the invention is therefore to provide a cathode material on the basis of a lithium manganese oxide spinel which is as free as possible from the abovementioned deficiencies and can be used in lithium batteries to be operated at 3V or 4V.